Abstract:
A number of synthetically useful ring systems can be prepared via the
intramolecular insertion of a metal-stabilized carbenoid into a heteroaromatic systems.
The chemical outcome of these reactions are dependent not only on the nature of the
heteroatom but also on the length of the aliphatic tether linking the carbenoid moiety with
the aromatic fragment. Our work with furanyl and thienyl systems containing a single
methylene tether have allowed for some rather atypical chemistry. For example, treatment
of l-diazo-3-(2-thienyl)-2-propanone (6) with catalytic rhodium (II) acetate yields 5,6-
dihydro-4^-cyclopenta[Z>]thiophen-5-one (3) while, the isomeric l-diazo-3-(3-thienyl)-2-
propanone(15) gives a spiro-disulphide (20).
Novel chemistry was also exhibited in the analogous furanyl systems. While
treatment of l-diazo-3-(3-furanyl)-2-propanone (52) with Rh2(OAc)4 resulted in the
expected 2-(4-Oxo-2-cyclopentenyliden)acetaldehyde (54), isomeric l-diazo-3-(2-
furanyl)-2-propanone (8) undergoes vinylogous Wolff rearrangement to give a mixture of
6a-methyl-2,3,3a,6a-tetrahydrofuro[2,i-^>]furan-2-one (44) and 2-(2-methyl-3-furyl)acetic
acid (43).
Rhodium acetate catalyzed decomposition of l-diazo-3-(3-benzofuranyl)-2-
propanone (84) and l-diazo-3-(2-benzofuranyl)-2-propanone (69)also allows for
vinylogous Wolff rearrangement, a chemistry unseen in benzofuranyl systems with longer
tethers. A number of interesting products were isolated from the trapping of intermediate
ketenes. Decomposition of l-diazo-3-(3-benzothienyl)-2-propanone (100) resulted in the
formation of 2,3-dihydro-l//-benzo[^]cyclopenta[^thiophen-2-one (102). However, in
addition to (102), a dimer was also generated from the decomposition of l-diazo-3-(2-
benzothienyl)-2-propanone (109).
The insight into the mechanistic underpinnings of the above reactions are provided
by molecular modeling at a PM3 level.
